Commit 80188b0d authored by Dave Chinner's avatar Dave Chinner Committed by Dave Chinner
Browse files

percpu_counter: batch size aware __percpu_counter_compare() 



XFS uses non-stanard batch sizes for avoiding frequent global
counter updates on it's allocated inode counters, as they increment
or decrement in batches of 64 inodes. Hence the standard percpu
counter batch of 32 means that the counter is effectively a global
counter. Currently Xfs uses a batch size of 128 so that it doesn't
take the global lock on every single modification.

However, Xfs also needs to compare accurately against zero, which
means we need to use percpu_counter_compare(), and that has a
hard-coded batch size of 32, and hence will spuriously fail to
detect when it is supposed to use precise comparisons and hence
the accounting goes wrong.

Add __percpu_counter_compare() to take a custom batch size so we can
use it sanely in XFS and factor percpu_counter_compare() to use it.

Signed-off-by: default avatarDave Chinner <dchinner@redhat.com>
Acked-by: default avatarTejun Heo <tj@kernel.org>
Signed-off-by: default avatarDave Chinner <david@fromorbit.com>
parent 74f9ce1c

include/linux/percpu_counter.h

+12 −1
Original line number Diff line number Diff line
@@ -41,7 +41,12 @@ void percpu_counter_destroy(struct percpu_counter *fbc); 
void percpu_counter_set(struct percpu_counter *fbc, s64 amount);

void __percpu_counter_add(struct percpu_counter *fbc, s64 amount, s32 batch);

s64 __percpu_counter_sum(struct percpu_counter *fbc);

int percpu_counter_compare(struct percpu_counter *fbc, s64 rhs);

int __percpu_counter_compare(struct percpu_counter *fbc, s64 rhs, s32 batch);



static inline int percpu_counter_compare(struct percpu_counter *fbc, s64 rhs)

{

	return __percpu_counter_compare(fbc, rhs, percpu_counter_batch);

}



static inline void percpu_counter_add(struct percpu_counter *fbc, s64 amount)

{
@@ -116,6 +121,12 @@ static inline int percpu_counter_compare(struct percpu_counter *fbc, s64 rhs) 
		return 0;

}



static inline int

__percpu_counter_compare(struct percpu_counter *fbc, s64 rhs, s32 batch)

{

	return percpu_counter_compare(fbc, rhs);

}



static inline void

percpu_counter_add(struct percpu_counter *fbc, s64 amount)

{

lib/percpu_counter.c

+3 −3
Original line number Diff line number Diff line
@@ -197,13 +197,13 @@ static int percpu_counter_hotcpu_callback(struct notifier_block *nb, 
 * Compare counter against given value.

 * Return 1 if greater, 0 if equal and -1 if less

 */

int percpu_counter_compare(struct percpu_counter *fbc, s64 rhs)

int __percpu_counter_compare(struct percpu_counter *fbc, s64 rhs, s32 batch)

{

	s64	count;



	count = percpu_counter_read(fbc);

	/* Check to see if rough count will be sufficient for comparison */

	if (abs(count - rhs) > (percpu_counter_batch*num_online_cpus())) {

	if (abs(count - rhs) > (batch * num_online_cpus())) {

		if (count > rhs)

			return 1;

		else
@@ -218,7 +218,7 @@ int percpu_counter_compare(struct percpu_counter *fbc, s64 rhs) 
	else

		return 0;

}

EXPORT_SYMBOL(percpu_counter_compare);

EXPORT_SYMBOL(__percpu_counter_compare);



static int __init percpu_counter_startup(void)

{

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